code: def validate_llm(cls, llm):
      if llm is None or not isinstance(llm, LLM or LangchainLLM):
          raise LLMNotFoundError("LLM is required")
      return llm
  
file_path: pandas-ai/pandasai/schemas/df_config.py
imports: from pydantic import BaseModel, validator, Field
  from typing import Optional, List, Any, Dict, TypedDict
  from pandasai.constants import DEFAULT_CHART_DIRECTORY
  from ..llm import LLM, LangchainLLM
  from ..exceptions import LLMNotFoundError
  from ..helpers.viz_library_types.base import VisualizationLibrary
label_desc: Use image node_id 1 for calling the Config obj's underlying member method code with example usage: obj.validate_llm(cls, llm) and returns: llm